As the designs of portable radio frequency (RF) communication devices, such as cellular telephones, personal digital assistants (PDAs), WIFI transceivers, and other mobile communication devices evolve, such devices must be capable of adjusting transmitted power accurately over a relatively wide dynamic range. For example, in the emerging markets of 3G/3.9G, linear systems such as those that communicate in accordance with standards such as WCDMA, WiMAX, EUTRAN-LTE, and other non-constant envelope modulation methodologies, the requirements for those standards for accurate transmitted power control continue to present challenges.
In mobile communication systems, power control is implemented to ensure that the power level of communication signals arriving at a base station from various mobile devices are relatively the same. To accomplish this goal, the base station continuously monitors the received signal power from each mobile device communicating with the base station. The base station directs each mobile device to adjust the transmit power level depending upon its distance, data rate change, or channel condition. The third generation partnership project (3GPP) specification calls for a maximum output power for a mobile handset of 24 dBm. A minimum output power for a mobile handset is −57 dBm. The difference between 24 dBm and −57 dBm results in a dynamic range of transmitted power of 81 dB. Providing such a dynamic range in a mobile communication device is not a difficult task. However, the 3GPP specification further includes a specification for transmit power step tolerance that with smaller commanded step sizes step sizes in transmit power becomes more stringent. A transmit power step tolerance describes a range of acceptable power levels in response to a base station command communicated to the mobile communication device that directs the device to adjust its transmit power. Table I below illustrates the transmit power step tolerance in accordance with the 3GPP specification. For example, when the base station directs the mobile communication device to increase transmitted power by 3 dB, the mobile communication device is required to increase transmitted power by 1.5 dB to 4.5 dB. As indicated in the first line of Table I, when no change in transmitted power is commanded by the base station, the transmitted power from the mobile communication device is required to not increase or decrease by more than 0.5 dB.
TABLE ITRANSMITTED POWERPOWER STEP SIZE ΔP(DB)STEP TOLERANCE (DB)0±0.51±0.52±1.03±1.5 4 ≦ ΔP ≦ 10±2.011 ≦ ΔP ≦ 15±3.016 ≦ ΔP ≦ 20±4.021 ≦ ΔP±6.0
A conventional transmitter includes a combination of three variable attenuators. Generally, two of the variable attenuators are connected in series with a third variable attenuator connected between the first two attenuators in shunt to ground (e.g., a “T” attenuator). Attempts to control the output power by controllably adjusting the attenuation provided by the variable attenuators to provide the transmitted power step tolerance of the 3GPP specification are problematic in that they require factory calibration to adjust for production tolerances and temperature variation over a relatively wide range of operational temperatures for the separate independent attenuation stages.